Electrolytic solution for zinc plating

ABSTRACT

AN ELECTROLYTIC SOLUTION FOR ZINC PLATING CONTAINING ZINC ACETATE, ZINC CHLORIDE OR ZINC SULFATE; AMMONIUM CHLORIDE; SODIUM OR POTASSIUM GLUCONATE; BORIC ACID; AND GLOSSING AGENTS. EQUAL OR SUPERIOR UNIFORM DEPOSITABILITY, CHROMATE TREATABILITY AND CORROSION RESISTANCE CAN BE OBTAINED WITH THE USE THEREOF.

United States Patent 4,97 6 Int. Cl. C23b /12, 5/46 US. Cl. 204-55 R 14 (Ilaims ABSTRACT OF THE DISfiLOSURE An electrolytic solution for zinc plating containing zinc acetate, zinc chloride or zinc sulfate; ammonium chloride; sodium or potassium gluconate; boric acid; and glossing agents. Equal or superior uniform depositability, chromate treatability and corrosion resistance can be obtained with the use thereof.

This invention relates to an electrolytic solution for zinc plating. More particularly, it relates to a process for plating zinc on metal surfaces and an electrolytic solution therefor which contains (1) zinc acetate, zinc chloride or zinc sulfate; (2) ammonium chloride; 3) sodium or potassium gluconate, (4) boric acid and (5) glossing agents.

Acidic zinc sulfate electrolytic solutions, alkaline zinc cyanide electrolytic solutions, etc. have principally been used heretofore as electrolytic solutions for galvanic zinc plating. The alkaline zinc cyanide electrolytic solutions are considerably superior to the acidic zinc sulfate electrolytic solutions insofar as uniform depositability is concerned. Thus, when a material having a complicated shape is to be plated, alkaline zinc cyanide electrolytic solutions have usually been used. However, the alkaline zinc cyanide electrolytic solutions contain very toxic cyanides, making the disposition of the electrolytic solution a very troublesome matter. Furthermore, much expense and valuable space are required for the disposal of the waste solution.

One of the objects of the present invention is to provide an electrolytic solution which contains no toxic cyanides and is equal or superior in performance and effect to an alkaline zinc cyanide electrolytic solution.

Another object of the invention is to provide an electrolytic solution capable of giving an excellent corrosion resistance by means of a relatively simple operation and device.

A further object of the invention is to provide an electrolytic solution capable of producing a good glossy and smooth surface.

A still further object of the present invention is to provide an eflicacious and advantageous process for the plating of zinc.

' These and other objects and advantages of the present invention will become apparent to one skilled in the art from a consideration of the following specification and claims.

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In accordance with the present invention, it has been found that the objectives outlined above are attained by using for the zinc plating an electrolytic solution comprising (1) zinc acetate, zinc chloride or zinc sulfate, (2) ammonium chloride, (3) sodium or potassium gluconate, (4) boric acid and (5) glossing agents.

The zinc ion concentration employed in the electrolytic solution of the present invention is 10-100 g./l. (grams per liter), preferably 20-50 g./l. The amount of zinc ion is closely related to the uniform depositability and cathode current efficiency which can be obtained, and when the zinc ion is used in excess, the uniformity of the deposit tends to worsen and become undesirable. As noted above, the zinc salts used in the present invention are zinc acetate, zinc chloride and zinc sulfate.

The ammonium chloride concentration used in the electrolytic solution is preferably 30-150 g./l. In this case, the ammonium chloride is used as a current-passing assistant, and when the ammonium chloride is used in excess amounts, the deposited zinc surface tends to become worse, with the result that the corrosion resistance also tends to deteriorate in quality. In addition, the adhesiveness of the deposited zinc to metal surface tends to become poor when too much or too little of an amount of ammonium chloride is used.

The concentration of potassium or sodium gluconate used in the electrolytic solution is preferably 30-300 g./l. More specifically, the concentration of sodium or potassium gluconate is preferably 180-220 g./l. in the case of zinc acetate, -120 g./l. in the case of zinc chloride and -140 g./l. in the case of zinc sulfate. The addition of sodium or potassium gluconate to the solution makes it possible to obtain a deposited zinc surface having a uniform gloss distribution. A film of chromate having a good corrosion resistance can be readily formed on the deposited zinc surface by chromate treatment. The adhesiveness of the deposited zinc to metal surface tends to become worse when too small of a quantity of gluconate is used.

The boric acid concentration is preferably 10-100 g./l. more preferably 15-25 g./l. The boric acid acts as a pH buffering agent. By adding H BO to the electrolytic solution, the gloss of the deposited zinc can be much improved.

The pH of the solution is preferably kept at 2 to 7, more preferably 4-5. The pH is closely related to the adhesiveness of deposited zinc to metal plate. The adhesiveness tends to become poor when too high or too low of a pH is used. A too low pH increases the solution of the zinc anode, and control of the electrolytic solution becomes difiicult.

Glossing agents which may be employed in the solution include, for example, gelatin, glue, dextrin, polyethylene glycol and triethanolamine.

Examples of the present invention are shown in the following table. In addition, the conventional electrolytic solution of alkaline zinc cyanide is also shown in the table as a comparison. These examples are given merely as illustrative of the present invention and are not to be considered as limiting.

5 conate, and from 10 to 100 grams per liter of boric acid, for a sufiicient period of time to plate zinc metal on said metal surface; said metal surface acting as a cathode.

11. The method of claim 10 in which the electrolysis is carried out at a cathode current density of about 3 amperes per square decimeter at a temperature of about 35 C.

12. The method of claim 11 in which said solution has a pH of 4 to 5.

13. The method of claim 10 in which said aqueous solution further contains an effective amount of at least one glossing agent selected from the group consisting of gelatin, glue, dextrin, polyethylene glycol and triethanolamine.

14. The method of claim 10 in which said metal surface 15 is on a ferrous metal.

References Cited UNITED STATES PATENTS OTHER REFERENCES W. E. Hughes: Modern Electro-plating, Oxford Tech. Pub., Henry Frowde and Hodder and Stoughton, p. 73,

GERALD L. KAPLAN, Primary Examiner 

